Drawing a Line on a Form

As an addition to Yusuf's answer, You would have to incorporate the use of the Mousedown and Mouseup handlers on the form. I won't lead you by the hand because I think you would benefit from figuring this out from the hints.
You need to get a starting point and keep it around. When the user releases the button you will need to permanently draw the line on the form. The process of actually drawing the line is well described in Yusuf's answer so I won't repeat that part here.

Look at this MSDN sample code.

System.Drawing.Pen myPen;
myPen = new System.Drawing.Pen(System.Drawing.Color.Red);
System.Drawing.Graphics formGraphics = this.CreateGraphics();
formGraphics.DrawLine(myPen, 0, 0, 200, 200);
myPen.Dispose();
formGraphics.Dispose();

Source: See link above

You can adapt this code to suit your needs:

using System.Drawing;
using System.Windows.Forms;
public partial class Form1 : Form
{
    private bool isMouseDown = false;
    private Point startPoint;
    private Point endPoint;
    public Form1()
    {
        InitializeComponent();
        MouseDown += new MouseEventHandler(Form1_MouseDown);
        MouseUp += new MouseEventHandler(Form1_MouseUp);
        MouseMove += new MouseEventHandler(Form1_MouseMove);
    }
    private void Form1_MouseDown(object sender, MouseEventArgs e)
    {
        if (e.Button == MouseButtons.Left)
        {
            startPoint = e.Location;
            isMouseDown = true;
        }
    }
    private void Form1_MouseUp(object sender, MouseEventArgs e)
    {
        if (e.Button == MouseButtons.Left)
            isMouseDown = false;
    }
    private void Form1_MouseMove(object sender, MouseEventArgs e)
    {
        if (isMouseDown)
        {
            endPoint = e.Location;
            Invalidate();
        }
    }
    protected override void OnPaint(PaintEventArgs e)
    {
        base.OnPaint(e);
        using (Pen pen = new Pen(ForeColor))
        {
            e.Graphics.DrawLine(pen, startPoint, endPoint);
        }
    }
}